Within certain technical fields, it is desirable in a simple manner to effectively separate two air or gas chambers from each other, where the partition is movable, although not always rectilinearly (not occuring along an axis common both chambers).
Such a need is i.a. evident at those air governing means, which are now being introduced at the recovery furnaces for the cellulose industry, i.e. steam boilers burning waste liquor from the pulping process.
This particular type of fuel, i.e. the waste liquor, tends to clog the passages supplying the combustion air to the furnaces, and of which there is a great number at the furnace, by fused residues (alcali salts), which are flowing down along the walls, or are thrown into the passages by the turbulence of the combustion gases.
The air governing means mentioned above includes an axially displaceable sleeve having a circumference compatible with, but preferably slightly smaller than the basic cross sectional area of the air port. The entrance to such a passage increases in the direction away from the furnace, and the sleeve is hermetically, but resiliently, connected to the outer wall of the plenum chamber supplying the passages with air. By adjusting the axial position of the sleeve so its free end extends more or less into the tapering mouth of the passage, it is possible to determine the effective passage area. The air will thus be forced to flow into the furnace through the annular clearance formed between the sleeve and the air passage port.
This type of governing means can be used for a further purpose; the sleeve may be intermittently pushed inwards, so that it will enter the furnace; in so doing it is possible to remove deposits upon the walls of the passage, whereupon it should be automatically retracted to its governing position.
During this scraping function, it may be advantageous to permit air to pass through the interior of the sleeve, as this will practically close the annular clearance normally surrounding itself. This further provides a safety air supply, should the sleeve be accidentally stuck in the passage. The interior of the sleeve shall thus normally, during the major part of its movement (=the extent of its governing function), be separated from the surrounding plenum chamber. At a certain most forward position, the interior of the sleeve should be made to communicate with the plenum chamber and then be sealed again, when the sleeve is retracted to governing position.
It is evident that this type of equipment cannot be manufactured with any high degree of precision. The sleeve may not move in a path exactly perpendicular to the outer wall of the plenum chamber, even if that was intended. The mechanism used for displacing the sleeve cannot, at reasonable costs, be manufactured with such exactness, that the longitudinal axis of the sleeve will always coincide with the designed longitudinal axis, or even, that the sleeve, after a "scraping stroke", will return along the same axis as the one it started from.